1. Field of the Invention
This invention relates to a three-phase flat-top waveform generator. More particularly, the invention relates to a three-phase flat-top waveform generator such as may be utilized in a pulse-width modulator supplying switching signals to operate a three-phase inverter.
2. Description of the Prior Art
Three-phase inverters are commonly used to supply loads requiring three-phase AC where input power is supplied to the inverter in the form of DC. These inverters are typically constructed having "poles" for each phase of the load. Generally, these poles each comprise a pair of serially connected semiconductor switches having respective anti-parallel diodes thereacross. The AC terminal of each pole is the node connecting the two semiconductor switches. Frequently, in the control of these inverters, a three-phase voltage reference signal is generated. This signal is typically fed to a pulse-width modulator which generates firing signals to control the semiconductor switches of each pole. Configurations of this type are illustrated in U.S. Pat. Nos. 4,959,602, 4,707,651, and 4,697,131.
If each phase of the three-phase voltage reference signal is a pure sine wave with a peak approximately equivalent to the range limit (+L to -L volts) of the modulator, the inverter output phase voltages will be sinusoidal and have an amplitude of one-half the value of the DC power supply. Under balanced conditions, the corresponding amplitude of the output line-to-line voltages is the square root of three divided by two (approximately 0.866) times the voltage of the DC power supply. If, however, the per-phase voltages have amplitudes greater than the range limit of the modulator, the line-to-line output voltages of the inverter are not increased.
If the AC load of the inverter is a three-wire system, however, zero sequence components can be introduced into phases of the reference signal without altering their line-to-line voltages. A signal can be generated comprising triplen harmonics (h=3,9,15, etc.) of the fundamental frequency such that, when added to each phase of the voltage reference, the range of the modulator can effectively be increased for the fundamental frequency. Using this approach, the amplitude of the output line-to-line voltage can be increased to the full value of the DC power supply. This technique is often referred to as "flat-topping" because it results in the voltage reference signal taking a clipped appearance at the crest of each phase.
In the case of a simple pulse-width modulated ("PWM") system having only voltage and frequency control, it is possible to synthesize the desired triplen harmonics and mix them with the phases of the voltage reference signal in the correct proportion for each prevailing condition. However, if the output currents of the inverter are controlled using closed-loop techniques, then the excursions of the phase reference signals are not readily predictable. In this case it is difficult to generate the required zero sequence component by triplen harmonic synthesis. The current control loops in use generally do not have sufficient closed-loop bandwidth to provide the required flat-topping by their own action. A similar problem arises in any situation where changeable three-phase voltages must be controlled subject to flat-topping.